1. Field of the Invention
The present invention relates to an aqueous ink having excellent ejection stability and when used as an ink for ink jet recording, free of occurrence of nozzle clogging.
2. Description of Related Art
The ink jet recording system is a system of obtaining a letter or an image by ejecting ink droplets from very fine nozzles directly to a recording medium and adhering these to the medium. In recent years, a multi-color printer is prevailing as an output apparatus of a personal computer and the like. According to the ink jet recording system, the multi-color printing can be easily performed and therefore, this recording system is rapidly popularized in various uses. The working principle thereof includes many systems such as deflection system, cavity system, thermo-jet system, bubble-jet system and thermal ink jet system. However, in any ink jet recording system, ink droplets must be continuously and stably ejected from fine nozzles. Accordingly, the ink for ink jet recording in required not only, similarly to normal ink, to provide a recording image having high quality, high fastness (e.g., water resistance, light fastness) and the like but also to ensure high ejection stability and no clogging of nozzles.
The ink for ink jet recording is in many cases an aqueous ink in view of its handleability, safety and the like. The aqueous ink can be roughly classified into a dye ink and a pigment ink. With respect to the dye ink, for example, JF-A-8-60053 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a dye ink containing an organic amine such as tripropanolamine, and states that this ink has excellent ejection stability. On the other hand, the pigment ink has excellent image fastness as compared with the dye ink and it is demanded to develop an ink for ink jet recording by making good use of this property. However, the pigment ink in inferior to the dye ink in the ejection stability and has a problem in that clogging of nozzles readily occurs accompanying the precipitation and aggregation of the pigment. Therefore, improvement of the ejection stability of the pigment dye is a technical problem which must be solved by all means.
With an attempt to improve the ejection stability of pigment ink, for example, JP-A-6-264016 discloses an ink for ink jet recording, which comprises a pigment, a polymer dispersant, a water-soluble organic solvent, a drying inhibitor and two or more organic amines. This pigment ink comprises aminomethylpropanol which is an organic amine compound having a propanol skeleton capable of imparting excellent ejection stability, however, due to the action of the polymer dispersant, the defoamability may decrease to cause bubbling and in turn, the ejection stability may decrease. Furthermore, in the case of this pigment ink, when the pigment content is increased to elevate the printing density, the polymer dispersant content must be increased in proportion thereto, however, the polymer dispersant even in a small amount is liable to increase the viscosity of ink and therefore, cannot be contained in a large amount in the ink. From these reasons, this pigment ink cannot be so much increased in the pigment content, as a result, the printing density cannot be sufficiently elevated.
The pigment ink described in JP-A-6-264016 is so-called a slow penetration-type ink which is less wettable to paper due to its high surface tension, therefore, this ink has a problem in that the drying time is long. Furthermore, when this pigment ink is used for multi-color printing, ink droplets on the recording medium may be connected with each other in the boundary of different colors to cause mixing of colors called bleeding and incur reduction in the image quality. Thus, this pigment ink is not suitable for multi-color printing. In addition, this pigment ink cannot be applied to the printing on a non-absorptive recording medium such as OHP sheet and restricted in the range of recording mediums to which the pigment can be applied.
Accordingly, an object (object A) of the present invention is to provide an aqueous ink composition favored with excellent ejection stability almost free of bubbling, prevented from occurrence of nozzle clogging on use as a ink for ink jet recording, capable of satisfactorily elevating the printing density, and facilitated in the handling.
Another object (object B) of the present invention is to provide an aqueous ink composition favored with excellent ejection stability, prevented from occurrence of nozzle clogging on use as an ink for ink jet recording, reduced in the drying time, difficult of bleeding and broadened in the range of recording mediums to which the ink composition can be applied.
A still other object (object C) of the present invention is to provide an aqueous ink composition favored with excellent ejection stability and on use as an ink for ink jet recording, prevented from occurrence of nozzle clogging.
Other objects and affects of the present invention will be apparent from the following description.
As a result of extensive investigations on the aqueous ink composition, the present inventors have found that when a surface-modified pigment capable of dispersing and/or dissolving in an aqueous medium without use of a dispersant is used as a pigment together with an organic amine compound having a propanol skeleton, an ink having excellent ejection stability almost free of bubbling and capable of being increased in the pigment content to a level of ensuring a sufficiently high printing density can be obtained (knowledge A).
Furthermore, as a result of extensive investigations on the aqueous ink composition comprising at least a coloring agent and water, the present inventors have found that by incorporating a specific organic amine compound, the above-described object C can be attained (knowledge C).
The present invention has been made based on the knowledge A, and the above-described object A has been achieved by providing an aqueous ink composition comprising at least a surface-modified pigment capable of dispersing and/or dissolving in an aqueous medium without use of a dispersant, an organic amine compound having a propanol skeleton, and water (hereinafter referred to as the first invention).
Also, the present invention has been made as a result of extensive investigations relating to the above described object B. The above-described object B has been achieved by providing an aqueous ink composition comprising a pigment, a dispersant, an organic amine compound having a propanol skeleton, a penetration accelerator, and water (hereinafter referred to as the second invention).
Furthermore, the present invention has been made based on the above-described knowledge C, and the object C has been achieved by providing an aqueous ink composition comprising at least a coloring agent and water, which comprises one or more organic amine compound selected from tho group consisting of hydroxyisopropyl dimethylamine, hydroxyisopropyl diisopropylamine, hydroxyisopropyl dibutylamine, dihydroxyisopropyl ethylamine, dihydroxyisopropyl isopropylamine, dihydroxyisopropyl butylamine, dihydroxyisopropyl hydroxyethylamine, dihydroxyethyl hydroxyisopropylamine, dihydroxyethyl-n-butylamtnie, hydroxyethyl diisopropylamine and hydroxyethyl di-n-butylamine (hereinafter referred to as the third invention).
According to the aqueous ink composition of the first invention, excellent ejection stability is ensured with almost no bubbling and when used as an ink for ink jot recording, fear of nozzle clogging is eliminated. Furthermore, the aqueous ink composition of the first invention does not contain a dispersant and therefore, is facilitated in the handling, for example, the pigment content can be increased to a level of providing a sufficiently high printing density as compared with an ordinary pigment ink containing a dispersant.
According to the aqueous ink composition of the second invention, excellent ejection stability is ensured and when used as an ink for ink jet recording, fear of nozzle clogging is eliminated. Furthermore, the aqueous ink composition of the second invention can provide a high-quality printed matter reduced in the drying time and having almost no ink blurring, can be freed from fear of bleeding on use for multi-color printing and can be favored with a broad range of recording mediums to which the ink composition can be applied.
According to the aqueous ink composition of the third invention, excellent ejection stability is ensured and when used as an ink for ink jet recording, fear of nozzle clogging is eliminated.